Ground penetrating radar (GPR) records of groundwater surface (GWS) reflections have been analyzed from 40 across-barrier profiles, totaling 50 km in combined length, taken from barrier spits and beach plains of the Columbia River littoral system. The barriers and beach plains host shallow fresh-water aquifers in the prograded beach deposits and abandoned foredune ridges, totaling 10–30 m in thickness. Study results demonstrate that GWS reflections could be traced continuously at subsurface depths of 1–15 m with the GPR 100 MHz and 50 MHz antennae using 400 V and 1000 V transmitters. Boreholes (62 in number) and lake water levels (24 in number) provide ground-truthing of the across-barrier GWS trends interpreted from the GPR profiles. The GWS rises in elevation (4–8 m above base level) under high, broad foredune-ridges and drops under interdune ridge valleys (1–3 m above base level). Continuous profiles of GWS demonstrate that lakes, ponds, and bogs of the barriers and beach plains are “windows” into the shallow coastal aquifer. The GPR records demonstrate that the GWS slopes either to seaward (0.003–0.04 gradient) or to landward (0.001–0.05 gradient) from divides under the largest, shore-parallel dune ridges in the barriers. The GWS gradients indicate that subsurface contaminant transport from the developed dune ridges will be intercepted by intervening lakes and ponds in the interdune-ridge valleys. The GPR records also establish the effect of drainage ditches in lowering GWS elevations (1–2 m) in sensitive wetlands located 100s of meters in distance from the constructed ditches.

Abstract High-resolution seismic profiling (300 km) and drill coring (17 sites to depths of 60 m below present sea level) in Grays Harbor, Washington, record the Holocene marine transgression and associated filling of the drowned-river basin (present surface area 290 km 2 ) in the convergent Cascadia margin. Seismic-reflection records are used to map the morphology of the ancestral Chehalis-Humptulips river valley (axial channel depth 60-70 m below present sea level) and to produce an isopach map of the basin fill (total volume 8.7 km 3 ). Drill coring establishes general textural sequences of sand and mud coarsening upward to sand and grave) in lower-bay reaches, and gravelly sand and sandy gravel fining upward to sand and mud in upper-bay reaches. Radiocarbon dating of core-sample wood, carbonate shells, and peaty muds yields a deposit depth-age curve in Grays Harbor beginning at 10,760±90 yrs (un-calibrated age in radiocarbon years before present, RCYBP) from 57 m depth below present sea level. Average basin sedimentation rates decrease from 1.2 cm/yr (10.5-7.5 ka) to 0.1 cm/yr (5.5-0 ka), following declining rates of eustatic sea-level rise in middle Holocene time. This trend corresponds to a four-fold decrease in average rates of basin-fill accumulation from 2.0 x 10 6 m 3 /yr (8.5-7.75 ka) to 0.5 x 10 6 m′/yr (5.5-0 ka), implying substantial river-sediment bypassing and/or diminished marine-sediment influx in the Grays Harbor basin during the latter half of the Holocene marine transgression.

Abstract The areal distributions of active-beach widths, eolian-dune fields, coastal-terrace heights and beach grain size are examined along a 1,000-km open coastline in the convergent Cascadia margin of the northeast Pacific. Beaches range from 10 to over 500 m in width in a total of 42 continuous beach segments, which are considered proxies for littoral cells at least 2 km long. Tectonic upwarping has exposed coastal-terrace deposits (1–30 m thick) and/or underlying bedrock to elevations as much as 120 m above mean sea level. The regional distributions of short-beach segments (average of 8–12 km long) in northern Washington and northernmost California—southern Oregon correspond to areas of maximum variation in terrace height, reflecting relatively rapid rates of tectonic vertical deformation. The three largest beach segments, 65 to 165 km long, are associated with major dune fields, and correspond to three of the largest river sources, namely the Eel, Columbia and Umpqua Rivers. Tectonic downwarping of inner-shelf sediment sinks might account for narrow, short-beach segments associated with the Klamath and Rogue Rivers. Six beach segments in central Oregon are isolated from either major river or high-terrace sand sources, and are assumed to be derived from trasgressive shelf sands. However, the observed longshore variability in these beach deposits indicates that pre-existing distributions of offshore sands were not uniform along the coast. Finally, longshore trends in increasing beach width, increasing dune development and/or decreasing beach grain size to the north in about one-quarter of the analyzed beach segments suggest a small net northward transport. However, inconsistent trends of all three variables in the remaining segments confirm the occurrence of short-term conditions responsible for reversing longshore transport in littoral segments effectively bounded by prominant headlands.

Abstract Surface sediment samples from 87 locations within the Gulf of California were analyzed in order to establish the supply and dispersal processes of the fine-grained sediments in this marine rift basin. Q-mode factor analysis of sediment bulk chemistry indicates that the surface sediments can be described in terms of four end-member sources, including terrigenous clastics, biogenic opal, biogenic carbonate, and manganese-rich components. The terrigenous factor accounts for as much as 80 to 90% of the total sedimentation on the eastern margin of the central and southern Gulf, but represents less than 40% of the total sediment on the western margin of the central and southern Gulf. Mass accumulation rates of the terrigenous component in nine laminated cores taken in the Gulf are estimated to range from 168 to 265 g/m 2 /yr (Guaymas Basin), 168 to 651 g/m 2 /yr (southeastern slopes), and 63 to 88 g/m 2 /yr (western slopes). Decreasing mass accumulation rates of the terrigenous component from the eastern to the western slopes of the southern and central Gulf account for the low content of biogenic components to the east (dilution by terrigenous silt and clay). The east-west trend in accumulation rates also indicates a dominant eastern (mainland) source of terrestrial sediment supply to the Gulf. A minor manganese component, of hydrothermal origin, is associated with basin-floor sediments of the Guaymas Basin in the central Gulf. X-ray diffraction studies of two size fractions—clay (2-4 μm) and silt (16-32 μm)—identify distinct mineralogical provinces within the Gulf based on quartz:feldspar ratios and on anorthite content of the plagioclase feldspars. The clay fractions throughout the northern Gulf (north of Tiburon Island) are dominated by Colorado River origins. In contrast, Colorado River silt is restricted to only the northernmost Gulf, north of Angel de la Guarda Island, an area of strong tidal forcing. In the central and southern Gulf, the silt and clay fractions have similar mineralogical origins, which largely reflect the geologic provenances of the Mexican mainland. Only minor inputs of silt and clay from Baja California are found on the western shelves and upper western slopes of the southern Gulf region. Whereas the southern Gulf shows dominant northeast-southwest (across-Gulf) mixing gradients, the central Gulf generally shows southeast-northwest (along-Gulf) mixing gradients. River sediment dispersal by Gulf surface and geostrophic currents is thought to predominate in the southern Gulf, where river runoff is high along the eastern margin. By comparison, eolian transport of terrigenous sediments from the arid northern mainland might play an important role in supplying sediment to the central Gulf basins, as suggested by Baumgartner et al. (1991b).

Abstract Substantial changes in mainland river discharge to the Gulf of California in historic time present a useful constraint in establishing major sources of fine-grained terrigenous sediment in the Gulf. A sharp decrease in water discharge of the Colorado River over the last 60 years, caused by extensive irrigation and damming, did not have a strong effect on the accumulation rate of terrigenous silt and clay in the Delfin Basin of the northern Gulf. Resuspension of fine-grained sediments from the broad, shallow shelf of the northern Gulf and / or eolian-transported silt and clay components from the mainland might be responsible for the constant supply and accumulation of sediments in the Delfin Basin. In the Guaymas Basin of the central Gulf, no consistent changes in the accumulation rate of terrigenous sediments are observed. The mineralogy of the sediments at the core site generally reflects mainland drainage sources which, like the Colorado River, were dammed and diverted for irrigation during the middle 1900s. Because the central Gulf lacks broad shelf reservoirs of sediment, the apparent constant supply of sediment to the Guaymas Basin implies eolian transport of sediment from mainland provenances to the east, as suggested by Baumgartner et al. (1991). Finally, cores from the southeastern slopes of the Gulf do show a significant decrease in the mass accumulation rate of terrigenous sediment since the 1930s. There are no indications of long-term climate (rainfall) changes in the southeast Gulf area during this period. We conclude that damming and diversion of the mainland rivers entering the southern Gulf are the major causes of the historic decrease in sediment mass accumulation rate in the eastern slopes of the southern Gulf.